The morphology of the renal tubules has changed during the phylogenetic development of the vertebrates. During this development animals may have required new renal functions in adapting to their changing environment. We intend to define renal function in primitive animals and to elucidate the evolutionary development of renal tubules which culminated in the kidney of man and mammals. We plan to determine first the characteristics of water and sodium transport in isolated perfused renal tubules from an euryhaline teleost fish. Permeability to water in the distal and collecting tubules appears to be much lower in fresh water (FW) teleost fishes than in marine species. Thus, FW teleosts excrete as urine the excess of water which invades their body fluids, and this renal tubular function helps them adapt to the hyposmotic environment. Euryhaline teleosts appear to adapt either to FW or sea water by flexibly changing tubular permeability to water. Fish renal tubules will be isolated and perfused in a specially designed chamber. Water and Na fluxes will be measured by isotopic methods. With this preparation we can determine directly the mode of fluid and electrolyte transport in each segment of the renal tubule without interference from hemodynamic factors. Preliminary experiments in goldfish indicate that the fish tubule is dissectable and perfusable. Neurohypophysical hormones, angiotensins, cortisol, and prolactin produce diuresis and/or natriuresis in bony fishes. We plan to define whether these hormones have a direct action on renal tubules using the isolated perfused tubule preparation. Fish nephrons are composed of various structures depending on their phylogenetic stage and they provide useful experimental models. These comparative studies may reveal aspects of original patterns of tubular function and may lead to better understanding of renal physiology in more advanced animals.